1. Field of the Invention
The present invention relates to an image reading apparatus, multifunction printer, and image reading method. Particularly, the present invention relates to an image reading apparatus, multifunction printer, and image reading method for optically reading an image original.
2. Description of the Related Art
In an image reading apparatus (scanner) having a CMOS solid-state image sensing element for a copying machine, scanner, camera, printer, or the like, and an image forming apparatus using the image reading apparatus, the CMOS solid-state image sensing element photoelectrically converts an original or target image, and generating image data. The analog image data is A/D-converted and undergoes digital processing, obtaining a digital image. A series of image reading processes is generally performed in synchronism with a scanner driving signal.
Recently, as the speed of image reading processing increases, the scanner apparatus faces the problem of increasing unwanted radiation (EMI: ElectroMagnetic Interference) of electromagnetic waves generated by the scanner apparatus. A regulatory/certification organization in each country defines the EMI radiation level, and the radiation needs to be suppressed to a regulated value or less. There are various methods to suppress unwanted radiation. As a means for suppressing harmonic radiation of a scanner-driving signal, a technique using SSCG (Spectrum Spreading Clock Generator) has been proposed.
SSCG is a technique of spreading the frequency of a driving signal in a predetermined modulation cycle. In an image reading apparatus using the SSCG technique, image noise sometimes occurs along with spread modulation during a series of reading processes. It is often difficult to suppress unwanted radiation while maintaining the image quality. Under such circumstances, various kinds of scheme have been made when applying the SSCG modulation technique to the scanner. For example, Japanese Patent Laid-Open No. 2007-081540 discloses an arrangement which switches on/off spread modulation in a predetermined period of a main scanning line signal when a CCD is used as a solid-state image sensing element. Spread modulation is switched on/off by arranging a control circuit, which controls a driving timing generation circuit to output an enable signal in order to control an SSCG module.
In a given apparatus, EMI radiation may be high in a circuit block including a CCD up to an A/D converter. When spreading a clock signal, high spreading degree contributes to decreasing the EMI radiation level but causes generating image noise. Japanese Patent Laid-Open No. 2002-209072 proposes a method using this characteristic to spread a clock signal at a spreading degree which differs between circuit blocks in accordance with the degree of influence on image noise.
However, if the driving signal of a CMOS solid-state image sensing element undergoes SSCG spread modulation, an electrical noise component may be captured in connection with spread modulation, degrading image quality. To suppress degradation of the image quality, for example, no SSCG spread modulation is adopted or the spread modulation degree is lowered. However, such a measure increases unwanted radiation. As summarized, it is very difficult for the conventional technique to suppress EMI while maintaining the image quality.
Further, it should be noted that recent scanner apparatuses are becoming less robust against external disturbance and electrical noise as the image reading speed increases, and the CMOS solid-state image sensing element and other modules are downsized. Especially in the use of the CMOS solid-state image sensing element, when transferring charges obtained by photoelectric conversion in each pixel at once to the holding capacitor, electrical noise such as external disturbance or bounce noise sometimes enters light reception data, and an unwanted periodical line, density unevenness, stripe, or the like appears in an image.